Cause
Reductant fluid past its shelf life
Reductant that has degraded with age or heat, so its concentration is outside specification. It is a precisely made solution with a limited life, typically around a year, and it degrades faster stored warm or in sunlight.
The quality sensor detects it and the system responds as it would to the wrong fluid — warnings, then derating, then the inducement sequence toward a no-start. So old fluid produces exactly the codes of a contaminated tank, and the diagnosis has to distinguish degraded from wrong.
A refractometer reads concentration directly and settles it in a minute, which is faster and more certain than any code. Check the container's date and where it was stored — fluid bought in bulk and kept through a hot summer is a common cause, and a partly used container left open degrades faster still. Drain, flush and refill with fresh fluid rather than topping up, since diluting good fluid into bad leaves the concentration wrong.
Typically an easy fix. Often worth checking before paying anyone to look.
Codes this can cause (2)
Codes where this is a likely explanation first, rather than merely a possible one.
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